Many modern medical procedures, both invasive and non-invasive, are performed with the assistance of medical imaging devices. A series of two-dimensional images generated from devices such as computed tomography (CT) or magnetic resonance (MR) scanners are used to generate virtual 3-dimensional models of regions of interest from subjects. These models are used in the planning, practice and execution of medical procedures subsequently performed on the patients. Image-guided medical procedures are generally accomplished by coordinating 3-D virtual models of subjects with reference points on the actual subjects undergoing the medical procedures.
Current approaches for image-guided medical procedures such as stereotactic biopsy or radiation therapy can be divided into two stereotactic approaches. Stereotactic is defined herein and is generally defined as a method utilizing 3-dimensional mapping to precisely locate areas in space suitable for guidance in medical procedures. Historically, stereotactic approaches have been used for procedures on areas of the brain and more recently for procedures in extra-cranial regions. The first approach uses a rigid frame secured to the subject, such as a human patient. The second approach is a frameless approach, in which fiducial landmarks, either derived from the patient's own anatomical features or applied to a surface of the patient, are used to coordinate the 3-D virtual patient model to the real world patient.
In the rigid frame approach, a stereotactic frame is rigidly attached to the patient by placing fasteners directly to the skull. A detailed 3-D image map is then created from CT, MR, or other 3-D imaging source. Fiducial markers placed on the stereotactic frame appear in the images and allow objects in the image to be related to the stereotactic frame. Hence, targets of interest, such as a tumor and its surrounding anatomy, can be described using the coordinates of the stereotactic frame as a reference. The patient is then transferred to the operating room for treatment with the stereotactic frame remaining in place, thus again using the stereotactic frame as a reference. Although this approach eliminates the error-generating step of registering the virtual model to the actual subject, this approach is invasive and results in discomfort for the patients.
In the frameless approach, the registration of the patient to the image-based operative model requires the identification of fiducial markers. Some fiducials are external markers applied to the patient prior to scanning and are kept in place until registration has been completed, while other reference markers are actually identifiable anatomic landmarks based on the patient's own anatomy. The identification of these fiducial points can be difficult and can add significant time to the operative procedure. Additionally, movement of the fiducials relative to internal anatomy can degrade the accuracy of the registration process and subsequently detract from the overall accuracy of the image-guided procedure.
Another method of obtaining a data set for stereotactic guidance is to provide a method of automatically mapping an image data set to a volume by defining the scan volume. In the case of large fixed equipment, such as a medical linear accelerator, the volume scanned by the imaging device can be mapped to a fixed point in the room. This is similar to the stereotactic mapping that is performed in carrying out procedures for ultrasonic guidance for the application of therapeutic radiation treatments. Alternatively, a reference can be applied to the scanning equipment and a second reference on the patient allowing the patient and the scanned data to be mapped to one another.
For non-spherically shaped tumors, as most tumors are, physicians generally attempt to fill the target with several radiation spheres of different sizes. This is referred to as a “sphere packing” treatment plan. Such methods require many computations in order to compute a radiosurgical plan dose distribution, and then require evaluation of the quality of the dose distribution.
In most systems a stereotactic ring apparatus or fiducial markers are applied to the patient to generate the images and then the planning process is initiated within minutes. However, planning might take two hours or more for a difficult case requiring about 15 to 20 spheres. During that time, the patient has to wait with the head ring attached to his or her head, or have fiducial markers remain disposed on his or her head. This places great restraints on the time necessary to derive an optimal plan as well as making expert consultation prohibitive.